The invention relates to an apparatus and method for deep rolling of crankshafts for increasing the fatigue strength thereof and, more particularly, to a rolling apparatus and method for crankshafts having split-pin bearings.
The principal of rolling crankshaft bearings in the fillet areas for increased fatigue strength has been known for many years. Rolling of fillet radii increases bending fatigue strength by applying compressive residual stresses into the areas below the surface of the material being rolled. The high-rolling forces however can, in certain instances, cause thin sections of metal at the sides of the bearings to bend over.
More particularly, there are radially extending side walls or fences on either side of a pin bearing for taking the side thrusts of the connecting rods of the engine pistons. As these fences extend radially beyond the outer surface of the pin bearing, the rollers can engage in the annular fillets therebetween and apply high compressive forces thereto. In this regard, the work rollers are typically angled or canted outwardly so that they can bear against these radial walls while being rolled in the fillets. As such, excessive applied force by the rollers can tend to distort or bend the radial wall portions if not properly controlled.
One way to avoid bending of the radial wall portions is to reduce the rolling force in the area where the bearings are in non-overlapping relation to each other, see, e.g. Japanese Patent Publication No. 60-24319 and U.S. Pat. No. 4,561,276. In this case, high pressure is applied by the tool actuator in the adjoinment area where the adjacent bearings are in overlapping relation and the actuator pressure is reduced in the non-overlap area where the wall is more prone to bending. The lower rolling force is sufficient to provide roll hardening to the crankshaft as it is mainly in the adjoinment area where the compressive residual strength is needed for strengthening this area of overlap between adjacent bearings.
In a conventional rolling tool, the rollers that impart the compressive residual forces are disposed generally opposite to each other and thus are angularly or circumferential aligned with respect to each other when engaged in the fillets on opposite sides of a crankshaft bearing. Therefore, when these rollers are applying the compressive rolling force to the bearing, be it at high or low levels, it is symmetrically applied to the fillet areas that are aligned and correspond to each other on either side of the bearing. Thus, when pulsing the rolling force during crankshaft rotation to avoid high pressures in the non-overlap areas prone to failure, the application of this varied force will occur symmetrically on each side of the pin bearing. In other words, the arcuate areas or surface portions of the opposite fillets that are rolled with a high force will be circumferentially aligned with each other about the bearing rolled. Similarly, those areas rolled with a lower force will likewise be aligned about the bearing.
However, a problem arises on a crankshaft having a split-pin bearing, such as on a V6-90 degree crankshaft, where the adjoinment overlap areas and non-overlap areas are not symmetrical or circumferentially aligned on each side of the pin bearings. This is because the split-pin bearings have one pin bearing that is offset arcuately from the other pin bearing, i.e. split, with these pin bearings lacking an intervening main bearing as is a common configuration for crankshafts. Thus, when taking an axial view of one of these split-pin bearings, the adjoinment overlap areas, as circumscribed by arcuate surface portions or segments of the bearings, located between it and the other pin bearing on one side thereof and the main bearing on the other side thereof will be shifted around the circumference of the one pin bearing so that these areas or arcuate surface portions are arcuately offset from each other and are not circumferentially aligned across the pin bearing from each other.
Accordingly, if a conventional rolling tool having opposite, aligned rollers are used in the fillets on either side of one of the split-pin bearings, any pulsing of the rolling force will not be able to be uniformly applied to both of the offset arcuate surface portions of the adjoinment areas on either side of the pin bearing. As such, having high forces applied by one of the opposite aligned rollers to the overlap arc surface portion at one side of one of the split-pin bearings will necessarily cause the other roller to apply high forces to a non-overlap surface portion on the other side of the pin-bearing where such surface portion is not circumferentially aligned or overlapping with the other surface portion that is being rolled with high forces. By having high rolling forces applied to bearing surface portions that circumscribe non-overlap areas of adjacent bearings, the risk of bending of the fence wall is increased, as previously discussed.
U.S. Pat. Nos. 5,495,738 and 5,575,167 disclose a two-stage process for rolling split-pin bearings in a manner that attempts to subject the adjoinment or overlap areas between adjacent bearings to a high level of rolling forces. In one stage, a pair of conventional tools having opposite, aligned rollers are employed, one on each split-pin bearing, so that their outer and inner rollers engage in respective outboard with inboard fillets on either side of the pin bearing they are to roll. These tools are independently operated so that the overlap areas on the inboard fillets of the pin bearings are rolled at higher pressures than the non-overlap areas. In the other stage, a modified tool is employed where a pair of tool housings are adjustably connected by a bearing unit therebetween. This modified tool has only outer work rollers with the inner rollers removed so as to only be able to apply rolling forces to the outboard fillets of each of the respective split-pin bearings without migration of the rollers off from the fillets in which they are engaged. Accordingly, in this stage, only the outboard fillets are rolled, either at a constant or variable pressure for roll hardening thereof.
For utilizing these two different tool units in the two stage rolling process of the ""738 and ""167 patents, it is disclosed that a single machine is retooled after one of the rolling stages or two machines are employed with one tooled with the conventional rolling tools and the other tooled with the double-housing tool. In either instance, there are significant inefficiencies introduced, both by the use of a two-stage rolling process for the split-pin journals and because of the use of different tooling units necessitating either retooling of a single machine between each stage of the split-pin bearing rolling operation or removing the crankshaft from one machine after the first stage and loading it into a second machine for second stage rolling.
Also, it is apparent that when rolling the inboard fillet of a split-pin bearing with conventional tools as taught by these patents, the rolling force applied to the outboard fillet on the other side of the split-pin bearing by the tools will not be properly located so that high forces are substantially confined to its adjoinment or overlap area with the main bearing adjacent thereto. Likewise, the low rolling forces will not be confined to the non-overlap area between the bearings in the outboard split-pin bearing fillet.
Accordingly, there is a need for a more efficient apparatus and method for roll hardening of crankshafts having split-pin bearings. More specifically, an apparatus and method are desired that do not require two stages for rolling the split-pin bearings for avoiding bending of the fence wall therebetween.
In accordance with the present invention, an apparatus and method are provided for roll hardening of crankshafts having split-pin bearings without requiring multiple rolling stages or operations therefor. In particular, the apparatus and method herein utilize a single tool unit that varies the rolling pressure on the fillets on either side of one of the split-pin bearings such that the areas needing strengthening are simultaneously rolled with a higher pressure than those areas at which bending of fence walls between adjacent bearings can occur with high pressure rolling, despite their arcuately offset orientation relative to each other. For this purpose, the tool unit has a pair of rollers rotatively housed at predetermined positions so that, when engaged against the opposite fillets of a bearing, they will be arcuately offset or spaced from each other about the bearing. Thus, the present tool allows the arcuately or circumferentially spaced surface portions of the respective fillets to simultaneously be subjected to high rolling forces for roll hardening thereof, whereas the arcuately or circumferentially spaced non-overlap areas at which the fence walls are prone to bend are simultaneously rolled with lower rolling forces so as to avoid bending of the radial walls. Accordingly, the crankshaft rolling apparatus and method herein employing the present tool enables split-pin bearings to be rolled with varied forces in a single rolling operation achieving significant cycle-time efficiencies over prior two-stage split-pin bearing rolling processes.
In contrast to prior tools which have their rollers aligned, the present rolling tool allows rolling pressures to be varied without causing the reverse effect in split-pin bearings from one side of the bearing to the other. In other words, the tool allows the pressure to be increased to simultaneously positively effect the areas prone to fatigue failure on both sides of the pin bearing despite their arcuate offset spacing from each other. At the same time, the tool also allows the pressure to be reduced so as not to create bending of the fence walls in those areas prone to bending on both sides of the pin bearing even though they are offset from each other.
Because the work rollers are no longer in line with each other axially across the bearing as in conventional tools, they will apply rolling force at offset areas on either side of the bearings in the fillets thereat. Since each work roller is to be applied at a circumferentially spaced position relative to the other in the respective fillets on either side of one of the split-pin bearings, it is preferable that each work roller have its own backup roller in the tool housing therefor. The tool housing can be elongated in a direction transverse to the axis of the held crankshaft and have an end from which the circumferentially offset or spaced rollers project that is configured to allow the rollers to engage in circumferentially spaced positions in the opposite fillets on either side of the split-pin bearing. In one form, the tool end from which the offset work rollers project has a V-shaped configuration so that it extends about the pin bearing to better enable the work rollers to be engaged at circumferentially spaced positions in the respective opposite fillets thereof. Accordingly, the present tool apparatus and method allow high rolling forces to be imparted to the areas where this is needed on either side of the split-pin bearing despite the offset orientation thereof.